1. Field of the Invention
This invention relates to a color image apparatus capable of forming a color image, such as a copying machine or a printer of an electrophotographic type, an electrostatic memory type or the like provided with a color measuring apparatus for automatically color-measuring an output image, a color measurement controlling method for the color image forming apparatus, and a storage medium and a program capable of being read by a computer.
2. Related Background Art
There has heretofore been a case where a color image forming apparatus or the like is provided with a color measuring apparatus for detecting image information from an image formed by the apparatus.
FIGS. 13A and 13B of the accompanying drawings illustrate the construction of a color measuring apparatus which can be utilized in a color image forming apparatus of this kind. FIG. 13A shows an example of the image detection thereof, and FIG. 13B shows an example of an image detection circuit (a circuit for converting the output current of a photodiode into a voltage). This example is an example of a sensor using a photodiode to detect reflected light from a toner.
In FIG. 13A, the reference numeral 201 designates a photodiode (sensor), and the reference numeral 102 denotes an LED which serves as a light source and which, as described below, illuminates a patch of toner with a constant amount of light.
The reference numeral 104 designates a toner patch which is an object of detection formed on a conveyed transferring material 1, and reflected light 206 from this toner patch 104 enters the photodiode 201, whereby a photocurrent is generated. The photocurrent is converted into a photoelectric conversion output (voltage signal) V203 by a resistor 202.
This photoelectric conversion output V203 reflects the amount of reflected light from the surface of a toner patch, in real time. A color measuring apparatus can be formed by a sensor using such a photodiode.
Specifically, as the LED light source 102, three LEDs of for example red (R), green (G) and blue (B), differing in light emission spectrum from one another, are provided to serve as light sources of respective colors, and the respective LEDs are caused to emit light independently of one another onto the toner patch which is the object of measurement, and the output of the sensor corresponding to each LED is obtained, whereby there can be obtained the R, G and B components of reflected light from the toner patch, and chromaticity such as L*a*b* or XYZ in color space processing or the like can be calculated.
FIG. 14 of the accompanying drawings is a block diagram showing the pixel construction of a line sensor of an accumulation type utilized in a conventional color image forming apparatus.
The reference numeral 204 denotes a sensor array comprising pixels 207 to 220. The reference numeral 205 designates a reading-out circuit, and the reference numeral 206 denotes a reset circuit. The pixels 207 to 209 and 220 are pixels of which the surfaces are shielded from light. The pixels 210 to 219 are a row of pixels responding to light.
The pixels 207 and 220 serve also as dummy pixels absorbing the unevenness of a sensor characteristic by their being located at the end portions.
Here, for simplicity, an example having ten pixels responding to light is described, but in practice the number of effective pixels is determined according to need. As the dark pixels, there has been shown an example in which there are 3 bits in the first half and 1 bit in the second half, but again the number of bits is increased or decreased depending on the degree of leakage of light between the pixels and the requirements of the particular actual system.
FIG. 15 of the accompanying drawings is a timing chart illustrating the operation timing of the line sensor of the accumulation type shown in FIG. 14.
First, a reset pulse 221 is applied to a port P1, thereby to reset the sensor 201, after which the reset is released and accumulation is started. During the accumulation, the accumulation capacitance (not shown) of the sensor 201 is charged with a photocurrent conforming to the amount of incident light.
However, the bits shielded from light have their accumulation capacitance charged with a dark current generated by the pixel portion. After accumulation for a predetermined time ta, a forwarding pulse 222 is applied to a port P2, whereupon the output of the sensor 201 is collectively forward to the reading-out circuit 205, and is outputted as an output signal 224 from a port P4 for each pixel on the basis of a shift pulse 223 inputted by a shift register in the reading-out circuit 205 through a port P3.
At this time, an output corresponding to the dark pixel 208 is defined as a dark time output and is subtracted from the outputs of the subsequent effective pixels, whereby there is obtained a signal in which an error due to the dark current of the sensor 201 has been corrected. The color measuring apparatus can also be formed by such a cumulative type sensor.
Specifically, for example, R, G and B filters are provided on the surface of the sensor, and the toner patch which is the object of measurement is illuminated by a light source like a white LED having a spectrum over the entire visible light area, and the output of the sensor 201 corresponding to each filter is obtained, whereby the R, G and B components of reflected light from the toner patch are obtained, and chromaticity such as L*a*b* or XYZ in the above-mentioned color space processing can be calculated.
FIG. 16 of the accompanying drawings is a schematic cross-sectional view illustrating the construction of a color image forming apparatus of this kind, and corresponds, for example, to the case of a four-set tandem type color laser printer.
Also, the color laser printer forms an electrostatic latent image by image light formed on the basis of an image signal in an image forming portion, develops this electrostatic latent image to form a visible image, further transfers this color visible image to a transferring material (a recording medium), and then fixes the color visible image.
In FIG. 16, the image forming portion is comprised of photosensitive drums 5Y, 5M, 5C, 5K, injection charging devices 7Y, 7M, 7C, 7K as primary charging means, developing devices 8Y, 8M, 8C, 8K and toner cartridges 11Y, 11M, 11C, 11K in respective stations juxtaposed by the number of developing colors, an intermediate transferring member 12, a sheet feeding portion, a transferring portion and a fixing portion 13.
Each of the photosensitive drums 5Y, 5M, 5C and 5K is constituted by an aluminum cylinder and an organic photoconductive layer applied to the outer periphery thereof, and is rotated by the driving force of a drive motor, not shown, being transmitted thereto, and the drive motor rotates the photosensitive drums 5Y, 5M, 5C and 5K counter-clockwise in conformity with an image forming operation.
Exposure light is sent to the photosensitive drums 5Y, 5M, 5C and 5K from scanner portions 10Y, 10M, 10C and 10K, and light from each of those scanner portions is selectively applied to the surfaces of the photosensitive drums 5Y, 5M, 5C and 5K, respectively, whereby electrostatic latent images are successively formed thereon.
As the primary charging means, provision is made of the four injection charging devices 7Y, 7M, 7C and 7K for charging the yellow (Y), magenta (M), cyan (C) and black (K) photosensitive drums in the respective stations, and the respective injection charging devices are provided with sleeves 7YS, 7MS, 7CS and 7KS.
As developing means, the four developing devices 8Y, 8M, 8C and 8K for effecting yellow (Y), magenta (M), cyan (C) and black (K) development in the respective stations are provided to visualize the above-mentioned electrostatic latent images, and the respective developing devices are provided with sleeves 8YS, 8MS, 8CS and 8KS. The respective developing devices are detachably mounted with respect to an apparatus main body.
The intermediate transferring member 12 is an endless belt member passed over a drive roller 18a and driven rollers 18b, 18c, is in contact with the photosensitive drums 5Y, 5M, 5C and 5K, is rotated clockwise during color image forming, and is sequentially subjected to transfer by the action of primary transferring rollers 6Y, 6M, 6C and 6K for the respective colors.
Transferring materials 1 are contained in a sheet feeding cassette 2 or a sheet feeding tray 3 as sheet feeding means (sheet feeding port), and the transferring materials 1 are conveyed one by one along a conveying path 25 constituted by a sheet feeding roller 4 and conveying rollers 24 and arrive at registration rollers 23. This is detected by an ante-registration sensor 19.
During image forming, the conveyance of the transferring material is stopped for a predetermined time by the ante-registration sensor 19 in timed relationship with the arrival of the color visible images on the intermediate transferring material 12 at a transferring area. The transferring material 1 is fed from the registration rollers 23 to the transferring area, and a secondary transferring roller 9 comes into contact with the intermediate transferring member 12 and nips and conveys the transferring material, thereby to superimpose and transfer the color visible images on the intermediate transferring member 12 onto the transferring material 1 one at a time.
The secondary transferring roller 9 is brought into contact with the intermediate transferring member 12 as indicated by a solid line as long as the color visible images are superimposed and transferred onto the intermediate transferring member 12, but it is brought to a spaced-apart position indicated by dotted line after the termination of the printing process.
The fixing portion 13 serves to fix the transferred color visible images while conveying the transferring material 1, and is provided with affixing roller 14 for heating the transferring material 1 and a pressure roller 15 for bringing the transferring material 1 into pressure contact with the fixing roller 14, as shown in FIGS. 13A and 13B. The fixing roller 14 and the pressure roller 15 are formed into a hollow shape, and contain heaters 16 and 17, respectively, therein. That is, the transferring member 1 bearing the color visible image thereon is conveyed by the fixing roller 14 and the pressure roller 15 and also have heat and pressure applied thereto, whereby the toners are fixed on the surface thereof.
After the fixing of the visible image, the transferring material 1 is discharged to a sheet discharging portion, not shown, by discharge rollers, not shown, thus completing the image forming operation. The discharge of the transferring material 1 from the fixing portion is detected by a fixed sheet discharge sensor 20.
Cleaning means 21 stores therein waste toners after the color visible images of the four colors formed on the intermediate transferring member 12 have been transferred to the transferring material 1.
Color misregister detecting means 22 forms a color misregister detection patch on the transferring material, detects the amounts of misregister among the colors in a main scanning direction and a sub-scanning direction, and applies feedback so as to finely adjust image data to thereby reduce color misregister.
When the above-described multicolor image forming apparatus is used, if fluctuation occurs in one or another portion of the apparatus due to a change in environment or the long-time use of the apparatus, the density and chromaticity of an image obtained will fluctuate.
Particularly, in the case of a color image forming apparatus of the electrophotographic type, even a slight density fluctuation may lead to the possibility of color balance being destroyed, and therefore, it is necessary always to keep constant density and gradation.
So, process conditions such as several kinds of exposure amounts and developing biases corresponding to absolute humidity and gradation correcting means such as a look-up table (LUT) are provided for the toner of each color, and on the basis of the absolute humidity measured by a temperature and humidity sensor, not shown, the then-applicable process conditions and gradation correction value are selected.
Also, in order that constant density, gradation and hue may be obtained even if, during the image forming process, fluctuation occurs to image forming colors in the respective portions of the apparatus with the fluctuation in environment, a toner image for density detection (hereinafter referred to as a “toner patch”) is formed on the intermediate transferring member with toner of each color, and this toner patch is detected by an optical sensor disposed at a location equivalent to the detecting means 22, and from the result thereof, feedback is applied to the process conditions such as the exposure amount and the developing bias and density control is effected, thereby to obtain a stable image.
However, in a color measuring apparatus using the sensor and the controlling method according to the prior art to obtain a stable image in the multicolor image forming apparatus, the following problems have been encountered in effecting the color measurement of the toner patch on the paper after fixing.
First, in order to obtain a stable image, it is necessary to form toner patches of various chromaticities on the transferring material, actually measure those chromaticities by the above-described sensor 201 to find the difference thereof from a desired chromaticity, and apply feedback to the process conditions.
Also, the reflectance of the toner patch is not uniform but varies from a high level to a low level, and the output of the sensor 201 fluctuates from a nearly saturated output to nearly a dark-time output. In the case of a patch which is high in density and low in reflectance, the output thereof becomes small and is buried in the quantization error during the AD conversion of the signal or the noise of a reading-out system, and a correct signal cannot be obtained.
Accordingly, in the case of a patch which is low in reflectance, color measurement accuracy becomes bad, and when the result thereof is to be fed back to the image forming apparatus in an effort to achieve color stabilization, there has been the problem that conversely the hue becomes unstable.